Make browser version, os, and os version always optional.

[airportyh]

Here are a few cases that work:

$ browserstack launch safari http://google.com
Launched job 2770009.
$ browserstack launch safari:6 http://google.com
Launched job 2770010.
$ browserstack launch safari:6.0 --os "OS X:Lion" http://google.com
Launched job 2770028.

Here are a few that don't:

$ browserstack launch safari:6 --os "OS X" http://google.com
Validation Failed - `os_version` can't be blank, `os_version` invalid
$ browserstack launch safari:6 --os "OS X:Lion" http://google.com
Validation Failed - `browser_version` invalid

This functionality will depend on logic in browseroverflow to intelligently select a browser based on the supplied spec.

[karlvr]

Toby, can you check those failure cases. They should work with the changes I submitted regarding the OS flag.